1. Field of the Invention
The invention relates to a vehicle differential apparatus and, more particularly, to a vehicle differential apparatus that has pinions that receive rotation drive force from a drive side, and side gears that mesh with the pinions so that the gear axis of the side gears is orthogonal to the gear axis of the pinions.
2. Description of the Related Art
A known vehicle differential apparatus has a differential case that rotates when it receives engine torque, a pair of side gears that are disposed side by side along a rotation axis line of the differential case, a pair of pinions that mesh with the pair of side gears, and a pinion shaft that supports the pinions (e.g., Japanese Utility Model Registration No. 2520728).
The differential case is provided with a housing space in which the two side gears and the two pinions are housed, and pinion insertion holes that communicate with the housing space. Besides, the differential case is provided also with a pair of axle shaft insertion holes that communicate with the housing space and that are open in a direction orthogonal to the axis line of the pinions.
Each of the two side gears is made up of a bevel gear that has a boss portion, and a gear portion, and is disposed movably along the rotation axis line of the differential case. Besides, the boss portion of each side gear is inserted into an axle shaft insertion hole, so that the side gears are supported freely rotatably within the differential case. Left and right axle shafts are inserted, respectively, into central holes of the two side gears. An inner peripheral portion of the central hole of each side gear is spline-fitted to its corresponding axle shaft.
Each of the pinions is a bevel gear that has a gear barrel portion whose outer periphery is not provided with gear teeth, and a gear toothed portion whose outer periphery is provided with gear teeth. The circumferential outer peripheral surface of the gear barrel portion of each pinion slides on an inner peripheral surface of a corresponding one of pinion insertion holes of the differential case. That is, the pinions are supported so as to be freely rotatable relative to the differential case. Each pinion has at its axial center position a shaft insertion hole through which a pinion shaft is inserted.
The pinion shaft is interposed between the two side gears, and is disposed in the housing space in the differential case. Besides, the pinion shaft is provided so as to be slidable on the inner peripheral surface of the shaft insertion hole of each pinion, and therefore supports the pinions freely rotatably.
Due to the foregoing construction, when torque is input from the vehicle's engine side to the differential case via the drive pinions and the ring gear, the differential case is rotated about the rotation axis line of the differential case. As the differential case is rotated, the turning force is transmitted to the pinions, and then is transmitted from the pinions to the side gears. Since the axle shafts are respectively linked to the side gears by spline fitting, the torque from the engine side is distributed to the axle shafts according to the situation of operation of the vehicle. Specifically, the torque from the engine side is transmitted to the left/right axle shafts via the drive pinions, the ring gear, the differential case, the pinions, and the side gears in that order.
When the pinions rotate, the outer peripheral surfaces of the gear barrel portions of the pinions slide on the inner peripheral surfaces of the pinion gear insertion holes of the differential case, and simultaneously the inner peripheral surfaces of the shaft insertion holes of the pinions slide on the outer peripheral surface of the pinion shaft. Therefore, friction occurs between the pinions and the differential case, and between the pinions and the pinion shaft. Due to this friction resistance, the rotation of the pinions about their own axes is suppressed, so that the differential rotation of the two side gears is restricted.
Besides, due to the rotation of the pinions, thrust force occurs on each side gear, moving the side gears in such directions as to move away from each other, so that each side gear is pressed against a peripheral edge of the opening of a corresponding one of axle shaft insertion holes. Therefore, friction occurs between each side gear and the peripheral edge of the opening of a corresponding one of the axle shaft insertion holes. This friction resistance, too, restricts the differential rotation of the two side gears.
In order to obtain sufficiently large differential restriction torque in the foregoing type of vehicle differential apparatus, it is important that the inner peripheral surface of the shaft insertion hole of each pinion and the outer peripheral surface of the pinion shaft frictionally slide on each other, with the axis line of each pinion being parallel to the axis line of the pinion shaft. Therefore, in order to minimize the clearance between the inner peripheral surface of the shaft insertion hole of each pinion and the outer peripheral surface of the pinion shaft, the inner peripheral surface of the shaft insertion hole of each pinion is machined or processed so that the diameter of the inner peripheral surface of the shaft insertion hole is uniform along the axis line of the pinion shaft (or along the axis line of the pinions).
Because of this, when the differential case rotates, the reaction force from the pinion shaft to the pinions which occurs due to the meshing between the pinions and the side gears act in a direction orthogonal to the axis line of the pinion shaft. While this state is maintained, the inner peripheral surface of the shaft insertion hole of each pinion frictionally slides on the outer peripheral surface of the pinion shaft.
However, according to the vehicle differential apparatus manufactured so that the size of the clearance between the inner peripheral surface of the shaft insertion hole of each pinion and the outer peripheral surface of the pinion shaft is uniform along the axis line of the pinion shaft, it is not determinable which parts of the inner peripheral surface of the shaft insertion hole of each pinion and the outer peripheral surface of the pinion shaft contact each other. That is, it is difficult to perfectly uniformly form the small gap between the inner peripheral surface of the shaft insertion hole of each pinion and the outer peripheral surface of the pinion shaft. In reality, this gap is non-uniform due to the machining or processing error. In the case where a near-center portion of the shaft insertion hole of a pinion contacts the outer peripheral surface of the pinion shaft, lubricating oil is not delivered to the entire area of the inner peripheral surface of the shaft insertion hole of the pinion, thus giving rise to a possibility of occurrence of wearing or seizing.